**Core Concept**
Right ventricular failure is a condition where the right ventricle of the heart is unable to pump blood effectively, leading to congestion in the systemic circulation. This can be caused by various factors, including pulmonary hypertension, left ventricular failure, and tricuspid valve disease. The commonest cause of right ventricular failure is often related to the increased pressure in the pulmonary circulation.

**Why the Correct Answer is Right**
The correct answer is related to the pathophysiology of pulmonary hypertension. Pulmonary hypertension is characterized by elevated pulmonary artery pressure, which increases the workload on the right ventricle. Over time, this can lead to right ventricular hypertrophy and eventually failure. The increased pressure in the pulmonary circulation is often caused by a decrease in the cross-sectional area of the pulmonary vessels, which increases the resistance to blood flow. This can be due to various factors, including chronic obstructive pulmonary disease (COPD), pulmonary embolism, or left ventricular failure.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because while left ventricular failure can cause right ventricular failure, it is not the commonest cause. Left ventricular failure typically causes right ventricular failure through a mechanism known as cor pulmonale, where the increased pressure in the left atrium leads to pulmonary congestion and eventually pulmonary hypertension.

**Option B:** This option is incorrect because while tricuspid valve disease can cause right ventricular failure, it is not the commonest cause. Tricuspid valve disease typically affects the right ventricle directly, whereas the commonest cause of right ventricular failure is related to the increased pressure in the pulmonary circulation.

**Option C:** This option is incorrect because while mitral valve disease can cause right ventricular failure, it is not the commonest cause. Mitral valve disease typically affects the left atrium and left ventricle, and the increased pressure in the left atrium can eventually lead to pulmonary congestion and pulmonary hypertension.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that right ventricular failure is often a consequence of left-sided heart failure, and the commonest cause is related to the increased pressure in the pulmonary circulation. This is often due to a decrease in the cross-sectional area of the pulmonary vessels, which increases the resistance to blood flow.

**Correct Answer: A. Pulmonary hypertension**